In a communication network (for example in the IEEE802.11 system (Wi-Fi)), which operates in the unlicensed band, a common problem that is encountered in a dense unplanned deployment is that when the transmitter sees a clear channel, it starts transmitting data to the receiver. However, the transmitter is generally unaware of the conditions at the receiver side. Assume that the receiver is actually encountering interference from an adjacent node. In this case, the receiving node may fail to decode the data received from the transmitter, due to the interference from the adjacent node. It is to be noted that such a situation will arise when the transmitter is beyond the transmission range of the interfering node while the receiver is within the range of the interfering node.
In the existing systems, the hidden node problem is addressed by defining a set of handshake signals known as RTS (Request To Send) and CTS (Clear To Send) such that when the transmitter intends to transmit something to an intended receiver, it sends a RTS which carries the identification of the intended receiver (the address of the receiver) besides carrying the identification of the transmitter. The receiver on hearing the RTS, sends back the CTS, which also includes the identification of the transmitter and the receiver (itself). The RTS and CTS also contain the estimated duration of the intended transmission. The transmitter starts the data transfer only after receiving the CTS. If the receiver does not see the channel as free (i.e. upon detecting unmanageable interference), then it does not sends the CTS and hence the transmission is carried out. The transmitter then waits for next opportunity to send the transmission and it again carries out the RTS/CTS handshaking.
On hearing the RTS, the nodes that are within the transmission range of the transmitter assume that the transmitter intends to transmit data to the receiver as indicated in the RTS for a particular duration, which is also included in the RTS. These nodes then back off (perform back off according to the back off algorithm as defined in the standard), in order to let the transmitter perform the intended transmission. Similarly on hearing the CTS, the nodes that are within the transmission range of the receiver understand that the receiver intends to receive data from the transmitter for a particular duration, as included in the CTS. These nodes then back off (perform back off according to the back off algorithm as defined in the standard), in order to let the receiver perform the intended reception. This helps in avoiding interference from the neighboring nodes of the transmitter as well as the receiver, thereby ensuring smooth communication.
The unlicensed band can support operation of cellular systems as well. However, presence of the hidden node problem can cause signal interference and in turn can result in bad user experience.